Optical sensing using the absorption bands of various gases in the visible or infrared (IR) wavelength range is an established method. The absorption may be measured in cavities with mirrors, so as to achieve an effective interaction length which is longer than the physical size of the cavity. This approach is limited by the optical losses in the mirrors. For IR, the source is often a broadband incandescent lamp. To get a spectral resolution, optical spectral analysis is then needed. Detectors can be thermal or semiconductor based photon detectors.
To make sensitive devices with a long optical path-length, either high quality mirrors must be used or the physical path, and hence the device size, must be long. For many applications, low gas flows and the large volume of the gas chamber limit the response speed of the sensor.
US 2014/0264030 A1 describes methods and apparatus for mid infrared sensing.
WO 2008/125797 A1 describes waveguide devices using evanescent coupling between waveguides and grooves.